The present invention relates to a coating material for tendons for prestressed concrete, which is to be coated on the surface of tendons used in post-tensioning prestressed concrete systems in order to protect the tendons from rust and corrosion and integrate the tendons with the concrete.
A post-tensioning system, which is a typical method for manufacturing prestressed concrete, is usually carried out as follows.
A metal sheath is positioned before placing concrete. A tendon, such as prestressing steel (prestressing steel wire, prestressing steel strand, prestressing steel bar, or the like) is inserted in the sheath. After the concrete hardens, tension is applied to the tendon. Then, cement milk or the like is injected between the sheath and the tendon for the purpose of protecting the tendon from rust and corrosion and integrating the tendon with the concrete.
According to this method, however, such operations as inserting a tendon in a sheath and injecting cement milk are troublesome and take much time and labor, which consequently raises the cost of manufacturing prestressed concrete.
Further, the space between the tendon and the sheath is very narrow and generally a tendon is positioned in a curved manner in the concrete, so that cement milk cannot be sufficiently injected along the whole length of the tendon. Accordingly, there is a likelihood that the tendon will corrode at portions where the cement milk is insufficiently injected.
In order to solve the abovementioned problems, various methods to preliminarily coat a coating material on the surface of a tendon have been proposed. These methods can be substantially divided into the following two groups:
.circle.1 one for obtaining antirust and anticorrosion effects, PA0 .circle.2 the other for preventing the tendon from attaching to the concrete.
An example of method .circle.1 is one which electrostatically coats the surface of a tendon of prestressing steel with an epoxy-resin as coating material. This method can obtain antirust and anticorrosion effects. However, because the coating material comes into a completely cured state after coating, this method needs such operations as inserting the tendon in a sheath and grouting to integrate the tendon with the concrete, which are required in the conventional post-tensioning system. Accordingly, it is impossible to lower the cost of manufacturing prestressed concrete. There has been proposed another method falling into the abovementioned groups .circle.1 , .circle.2 , which employs so-called unbonded prestressing steel which is coated with grease and then covered with polyethylene or the like. This method in which the abovementioned unbonded prestressing steel is positioned before placing concrete and tension is applied to the unbonded prestressing steel after the concrete hardens, has an advantage that the tension applied to the prestressing steel can be transmitted to the whole length of the prestressing steel because of the presence of the fluid grease between the prestressing steel and the concrete. As a result, a metal sheath used in the conventional post-tensioning system is not required, which naturally eliminates insertion of a tendon in a sheath, injection of cement milk and grouting, which are necessary operations for the conventional post-tensioning system. Accordingly, the disadvantage which causes the conventional post-tensioning system to be expensive can be eliminated. However, since grease as coating material does not harden, the tendon cannot adhere to the concrete forever, which consequently reduces the bending strength and the fatigue strength of the concrete.
In order to eliminate the abovementioned disadvantage of the method using unbonded prestressing steel, a method has been proposed which comprises coating a prestressing steel with thermosetting composition in an uncured state, positioning the steel similarly to the case of the abovementioned unbonded prestressing steel, and curing the coated thermosetting composition by heating the prestressing steel after applying tension to the prestressing steel by means of high-frequency heating or the like so as to make the prestressing steel adhere to the concrete. However, this method, which requires heating of the tensioned prestressing steel, is very dangerous. Further, it is impossible to accurately heat only a predetermined steel in a large-sized concrete construction. Accordingly, this method has disadvantages in that the prestressing steel cannot be made to adhere to the concrete over the whole length thereof.
In order to solve the abovementioned problems, an object of the present invention is to provide coating material for tendon for prestressed concrete which can provide prestressed concrete at a reduced cost and assure antirust and anticorrosion effects for the tendon and adherence of the tendon to the concrete.